1. Field of the Invention
The present invention relates to high-strength, high-damping-capacity cast iron having a high strength and high vibration damping capacity.
2. Description of the Related Art
Presently, noise is at a higher rank of complaints for seven major types of pollution, i.e., air pollution, water pollution, soil pollution, vibration, noise, land subsidence, and bad smell. Of complaints for noise, construction work noise accounts for a high proportion. Since complaints like these concentrate in urban areas, it is of urgent necessity to reduce noise of urban construction machinery. Also, as environmental friendliness tends to be regarded as important worldwide, the EU noise regulations including sales regulations are becoming stricter. As such, noise reduction cannot catch up to the enforcement of noise regulations by extending existing technologies. There is a tendency to view low-noise vehicles as a world-standard for vehicles to hereafter comply with the global trend of regarding environmental friendliness as important. Construction machines are already required to reduce noise to such an extent as that of automobiles, and attempts have been made to reliably reduce noise of engines, fans, mufflers, and the like. Hereinafter, noise reduction of the entire hydraulic system need be dealt with.
To accomplish noise reduction of a hydraulic system, it can be conceived to have the material of heavy machinery hydraulic parts possess vibration damping performance. However, flaky graphite cast iron, which exhibit vibration damping performance (a noise reducing effect), has too low a strength for applying to heavy machinery hydraulic parts made of cast-iron. Therefore, a material having strength relevant to that of conventionally used spheroidal graphite cast iron is necessary.
More specifically, in heavy machinery hydraulic parts, noise is generated at a control valve, motor cover, or the like, and this noise becomes relatively apparent as the engine noise of the heavy machine is reduced. All of such parts are made of spheroidal graphite cast iron or CV (Compacted Vermicular) graphite cast iron, and their strengths are 400 to 500 MPa. By contrast, it is difficult to obtain strength of 350 MPa or more for flaky graphite cast iron.
Patent documents 1 and 2 describe high-rigidity, high-damping-capacity cast iron having high vibration damping capacities. However, these cast iron are flaky cast iron, and therefore their strength is insufficient.
Patent document 3 describes cast iron containing fine-size graphite, which is obtained by adding a rare earth-Si-iron alloy. This cast iron described in patent document 3 is relevant to FC200-class cast iron, in which its vibration damping capacity has been improved without decreasing its strength. However, the strength of this cast iron is only about that of FC200.
Patent document 4 describes a cast iron material which exhibits excellent vibration damping capacity by having fine pores in addition to flaky graphite. In this cast iron material, the vibration damping capacity can be improved by increasing the porosity in a base structure. As a consequence though, the strength decreases as the porosity increases.
The object of patent document 5 is to obtain a cast iron material excellent in both vibration damping capacity and strength. This document describes that the vibration damping capacity is raised by dispersing steadite together with flaky graphite.
None of the high-damping-capacity cast iron described in patent documents 1 to 5 have strength of 400 MPa or more required for heavy machinery hydraulic parts of construction machines, however.